The present invention relates generally to the field of methods of and systems for capturing fingerprint images, and more particularly to an optical semiconductor fingerprint scanning device.
Fingerprint recognition has been suggested for use in many security applications, such as controlling access to buildings, computers, or the like. Fingerprint recognition systems enable a user to access the controlled facility without having to provide a device such as a keypad or card reader, and without having the user memorize a password, or other personal identification number.
The sensing device is an important part of a fingerprint recognition system. The quality of the fingerprint image that the sensing device produces will affect recognition capability and the amount of processing required for verification of the fingerprint.
Various technologies have been proposed for use in fingerprint sensing devices. One commonly proposed technology involves optical image detection. Examples of optical fingerprint detection devices are disclosed in Jensen, U.S. Pat. No. 4,784,484; Fishbine, et al., U.S. Pat. No. 5,467,403; and Giles, et al., U.S. Pat. No. 5,548,394.
The fingerprint sensing device captures the fingerprint image with an array of sensing elements. In one design, the fingerprint image is captured with a relatively large array. The user places the finger tip over the array, and the array is scanned to capture the fingerprint image.
Acceptable images can be captured using a resolution of about 500 dpi, which requires a sensing element size of about 50 microns. A large array that can capture a fingerprint image in a single scan is typically about 360 by 256. A large array device can capture many fingerprint features and minutiae, which leads to accurate fingerprint recognition. A large array device captures an entire fingerprint image in a single scan. Accordingly, the fingerprint image does not need to be reconstructed. However, the larger the array, the higher the cost of the device.
An alternative to large array devices are narrow array devices. In a narrow array device, the array has a first dimension that is about the width of a fingerprint and a length that is substantially less than the length of a fingerprint. The fingerprint image is captured as the user sweeps the finger tip over the narrow array as the array is scanned. On each scan, the narrow array captures a partial image or slice of the fingerprint. A regeneration algorithm assembles the slices into the complete fingerprint image.
One of the problems with narrow array devices is that the speed at which the finger is swept over the array is unknown. In order to reconstruct the fingerprint image, a pair of consecutive slices must have enough rows in common for them to be aligned by the regeneration algorithm. Thus, the fingerprint image must be over-sampled. Since different speeds and the speed at which a person moves the finger during any particular sweep is generally not uniform, the fingerprint image must be highly over-sampled.
The narrow array must be scanned at a relatively high clock rate to ensure that the fingerprint image is sufficiently over-sampled. The over-sampling required for accurate image reconstruction requires substantial memory buffer. The regeneration algorithm required for reconstructing the image requires processor resources. If reconstruction processing is done in the host computer, substantial memory and processing are required. If reconstruction processing is performed on the chip, then substantial memory and a digital signal processor (DSP) are required on the chip.
Thus, although a narrow array device is less expensive to build than a large array device, narrow array devices may be more expensive in terms of memory and processing resources.
The present invention provides a narrow array optical fingerprint detector that substantially eliminates over-sampling of the array, thereby eliminating reconstruction and additional memory requirements. The fingerprint detector includes an array of optical sensing elements. The fingerprint detector of the present invention measures the speed of a finger moving over the array and scans the array at a rate determined by the speed of movement of the finger.
In the preferred embodiment, the fingerprint measures the speed of finger movement with a transparent cylinder rotatably mounted adjacent the array. The transparent cylinder is mounted to engage the finger and rotate as the finger is swept past the array. A light chopper is mounted for rotation with the cylinder. A photo-sensor is mounted adjacent the light chopper. The photo-sensor produces a signal in response to light being chopped by the light chopper. The photo-sensor is operably connected to scanning circuitry. Each time the scanning circuitry receives a signal from the photo-sensor, the scanning circuitry scans the array.
The fingerprint detector of the present invention may also include wake-up and idle circuitry operably connected to the photo-sensor. The wake-up and idle circuitry places the fingerprint detector into an idle or standby state after a predetermined delay in receiving a signal from the photo-sensor. The wake-up and idle circuitry places the fingerprint detector into an active state in response to receiving a signal from the photo-sensor when the fingerprint detector is in the idle state.